<style>
    #lev1 {
        display:none;
    }
    #lev2 {
        display:none;
    }

</style>
<div class="box round first">
        <h2><?php echo __('Add Category'); ?></h2>
<div class="categories form block">
<?php echo $this->Form->create('Category'); ?>
    <fieldset>
    <?php
        echo $this->Form->input('title');
        echo $this->Form->input('lev', array('options' => array('1' => 1, '2' => 2, '3'=> 3), 'empty' => FALSE));
        echo $this->Form->input('sub', array('id' => 'CategorySub1', 'options' => $lev1, 'empty' => __('choose one'), 'div'=>array('id'=>'lev1')));
        echo $this->Form->input('sub', array('id' => 'CategorySub2','options' => $lev2, 'empty' => __('choose one'), 'div'=>array('id'=>'lev2')));
    ?>
    </fieldset>
<?php echo $this->Form->end(__('Submit')); ?>
</div>
</div>
<script type="text/javascript">
    window.onload = function() {
        var eSelect = document.getElementById('CategoryLev');
        var lev1 = document.getElementById('lev1');
        var lev2 = document.getElementById('lev2');
        var CategorySub1 = document.getElementById('CategorySub1');
        var CategorySub2 = document.getElementById('CategorySub2');
        eSelect.onchange = function() {
            if(eSelect.selectedIndex === 1) {
                lev1.style.display = 'block';
                lev2.style.display = 'none';
                CategorySub1.disabled=false;
                CategorySub2.disabled=true;

            }
            if(eSelect.selectedIndex === 2) {
                lev1.style.display = 'none';
                lev2.style.display = 'block';
                CategorySub1.disabled=true;
                CategorySub2.disabled=false;
            }
            if(eSelect.selectedIndex === 0) {
                lev1.style.display = 'none';
                lev2.style.display = 'none';
                CategorySub1.disabled=true;
                CategorySub2.disabled=true;
            }
        }
    }
  </script>